Local oscillator (LO) signals are required in various applications. In a cellular phone, for example, local oscillator signals are needed as input to a receiver mixer for downconverting a received radio frequency (RF) signal and as input to a transmitter mixer for upconverting a signal which is to be transmitted as radio frequency signal.
A local oscillator signal is usually provided by a voltage controlled oscillator (VCO) which forms a part of a phase-locked loop (PLL). For a cellular phone, generally two separate phase locked loops are used, a first one with a first VCO handling all frequency bands required for reception (RX) and a second one with a second VCO handling all frequency bands required for transmission (TX).
For illustration, FIG. 1 presents a conventional implementation of a PLL structure with two PLLs for a cellular phone. The PLLs are also referred to as synthesizer loops.
In the lower half of FIG. 1, a first PLL includes a locking unit 10 with a programmable reference input, a LO signal prescaler, programmable dividers and a phase detector. The output of the locking unit 10 is connected in a row to a charge pump 11, a loop filter 12, an all band TX VCO 13 having a frequency range of 3296 MHz to 3980 MHz and a transformer 14. This frequency range is suited for example for an LO signal for GSM (Global System for Mobile Communications) and WCDMA (Wideband Code Division Multiple Access) transmissions. The output of the transformer 14 is connected on the one hand via a first buffer 15 to the input of the locking unit 10, thereby closing the first PLL. The output of the transformer 14 is connected on the other hand via a second buffer 16 to an LO output 17 providing an LO signal for the TX chain (not shown) of the cellular phone.
In the upper half of FIG. 1, a second PLL is depicted. The second PLL has an identical structure as the first PLL and comprises thus as well a locking unit 20, a charge pump 21, a loop filter 22, a VCO 23, a transformer 24 and a first buffer 25 arranged in a loop. In this case, however, the VCO 23 is an all band RX VCO having a frequency range of 3476 MHz to 4340 MHz. This frequency range is suited for example for an LO signal for GSM and WCDMA receptions. The output of the transformer 24 is connected outside of the second PLL via a second buffer 26 to an LO output 27 providing an LO signal for the RX chain (not shown) of the cellular phone.
The two PLLs of the PLL structure operate in a well known manner. A reference signal having a programmed frequency is applied to the reference input of the respective locking unit 10, 20. Further, the current output signal of the respective VCO 13, 23 is provided via the respective transformer 14, 24 and the respective first buffer 15, 25 to an input of the respective locking unit 10, 20. The transformer 14, 24 is used for transforming the single-ended output provided by the discrete TX VCO 13, 23 to the differential signal which is often required by integrated PLL components. It is to be noted that in case of integrated VCOs, the respective VCO 13, 23 could have a differential output itself. In this case, the respective transformer 14, 24 would not be needed. The frequency of the VCO signal is first divided in the respective locking unit 10, 20 by the LO prescaler and then by the programmable frequency dividers. The phase detector compares the phase of the frequency divided VCO signal with the phase of the reference signal and outputs an error signal, which constitutes at the same time the output of the locking unit 10, 20. The PLL is locked when the two phases are equal, which implies that also the frequencies of the signals are equal.
For achieving or maintaining a locked state, the respective charge pump 11, 21 generates current impulses, the length of which are controlled by the output signal of the respective locking unit 10, 20. The generated current pulses are filtered by the respective loop filter 12, 22, which provides a corresponding control voltage to the respective VCO 13, 23 and thus takes care that the VCO 13, 23 generates a signal which is locked to a desired frequency. The frequency of the signals output by the VCOs 13, 23 can be changed by changing the factor in the programmable dividers in the respective locking unit 10, 20. The phase locked VCO signal is buffered by the respective second buffer 16, 26 and provided to the respective LO output 17, 27, which may also realize an output matching.
One aspect that has to be taken into account when designing a PLL structure for a cellular phones is the current consumption, which should be as low as possible in order to save battery power. The current consumption of the PLL structure is mainly determined by the quality of the VCO signals required by the respective RF and RF system specifications. The quality of the signal includes for example the phase noise, the signal to noise floor, the output power, the environmental stability, etc. Moreover, the current consumption of a VCO increases with the frequency band which has to be covered.
Different systems have different requirements and possibilities to lower the power consumption, which is of particular importance for cellular phones supporting different systems. A GSM/WCDMA multimode/multiband phone supports for example a GSM based system and as well a WCDMA based system. These multimode/multiband systems require a wide frequency band for their RX LO signal. GSM based systems in general require a higher signal quality for their RX LO signal than WCDMA based systems. WCDMA based systems, on the other hand, generally require a very low power consumption of the VCOs, because these have to be turned on almost all the time during a conversation.
Also within the same system, different phone usage conditions, like the network environment, require different purity from the VCO signal. More specifically, when no disturbing high-level signals are present, a lower VCO signal quality is required than in case such disturbing high-level signals are present.
In order to reduce the current consumption, there is the possibility to switch off a VCO and possibly an entire PLL. Whenever the cellular phone is not receiving an RF signal or not transmitting an RF signal, the concerned VCO and PLL can be switched off, while taking care that the PLL is settled at the right time. Usually, however, the PLL loops are powered all the time in the conversation mode of GSM.
In order to be able to use low performance VCOs with low power consumption in a WCDMA mode of a combined GSM/WCDMA cellular phone, it has further been proposed to employ three phase locked loops with three VCOs. One VCO is used in such a system for the RX and TX frequency bands needed for GSM, a second VCO is used for the RX frequency bands needed for WCDMA and a third VCO is used for the TX frequency bands needed for WCDMA. It is a disadvantage of this approach, though, that an additional VCO is required.
Another aspect that has to be taken into account when designing a PLL structure for a cellular phones is time critical situations. In a time critical situation, the frequency of an LO signal has to change quickly, while a PLL requires some time for settling to a newly set frequency.